Roaming management for client devices

ABSTRACT

Embodiments of a wireless roaming management system are disclosed, wherein the system is configured to manage roaming of a client device across access points. The system includes processor(s) and memory for storing signal strength data for access points operating on a local area network. In one embodiment, the system is configured to transfer a client device from one access point to another. For example, in response to receiving, from a source access point, a request for a roaming calculation for a client device connected to the source access point, the system can determine a target access point to transfer the client device based on the stored signal strength data and the roaming calculation. The system can then cause the client device to be disconnected from the source access point and connected to the target access point.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No.61/814,112, entitled “ROAMING MANAGEMENT FOR CLIENT DEVICES,” filed Apr.19, 2013, which is herein incorporated by reference in its entirety.

BACKGROUND

Many local area networks (LANs), such as found in homes or businesses,include multiple wireless access points and/or range extenders(collectively referred to as “access points” herein for convenience) inorder to increase wireless coverage over an area. Thus, a client deviceoperating on the LAN may connect to multiple access points as a usermoves around the area. For example, the user may connect to a firstaccess point in the bedroom and a second access point in the livingroom. During the course of a day, the user may move and switch accesspoints multiple times.

BRIEF DESCRIPTION OF THE DRAWINGS

Throughout the drawings, reference numbers may be re-used to indicatecorrespondence between referenced elements. The drawings are provided toillustrate example embodiments described herein and are not intended tolimit the scope of the disclosure.

FIG. 1 illustrates a block diagram of an embodiment of a roamingmanagement (RM) system that provides for client roaming between accesspoints.

FIG. 2 illustrates a block diagram of another embodiment of the RMsystem.

FIG. 3 illustrates a flow chart of an embodiment of a client transferroutine for a roaming manager of the RM system.

FIG. 4 illustrates a flow chart of embodiments of client transferroutines for access points of the RM system.

FIG. 5A illustrates an embodiment of a data structure for storing signalstrength data monitored by the RM system.

FIG. 5B schematically illustrates a network diagram of an example localarea network (LAN) that corresponds to the signal strength data of FIG.5A.

FIGS. 6A-B schematically illustrates a network diagram showing an accesspoint transfer from a first access point to a second access point.

FIG. 7 illustrates a representative network topology diagram associatedwith an access point (AP) relation table.

DETAILED DESCRIPTION

As users move across different access points, the resulting connectionsfrom user devices to respective access points may not be optimal. Forexample, client devices may be configured to retain a connection to afirst access point, even if the client device has moved to a locationwhere a second access point now has a stronger signal. Thus, the clientdevice may not have the best wireless signal available, which can resultin slow downs, broken connections, errors and other issues with theclient device's wireless connection. Thus, providing a system that isable to transparently, to the client device, handoff the client devicefrom one access point to another can be beneficial.

Embodiments of a roaming management (RM) system are disclosed, whereinthe system is configured to determine relative signal strengths ofaccess points, select the access point deemed capable of providing thestrongest signal to the client device, and force the client device toconnect to the selected access point.

In one embodiment, a roaming manager comprises one or more processorsand memory for storing signal strength data. The roaming manager isconfigured to identify a roaming determination trigger and, if a roamingdetermination is triggered, determine a target access point to service aclient device based on signal strength data. The roaming manager thencauses the client device to be disconnected from a current access pointand connect to the target access point.

Various aspects of the RM system will now be described with regard tocertain examples and embodiments, which are intended to illustrate butnot to limit the disclosure. Nothing in this disclosure is intended toimply that any particular feature or characteristic of the disclosedembodiments is essential. The scope of protection of certain inventionsis defined by the claims.

FIG. 1 illustrates a block diagram of an embodiment of a roamingmanagement system that provides for client roaming between accesspoints. In the embodiment of FIG. 1, the RM system 100 includes aroaming manager 105, which may be operating on a networking device(e.g., a router, gateway, switch, modem, access point, range-extender,etc.) or server, and a plurality of access points 110 a, 110 b (alsoreferred to as access point 110) or other wireless network accessdevices connected via a local network 115 (e.g., a LAN). There may beany number of access points depending on the desired coverage area forthe local network 115. In some cases, access points may bemulti-functional (e.g., a router/AP, AP/range-extender, etc.). In oneembodiment, each access point is configured to broadcast the sameservice set identification (SSID), though other embodiments may usemultiple SSIDs. The connections may be wired or wireless (e.g.,802.11a/b/g/n/ac). In one embodiment, the roaming manager 105 includes acalculation module 120 for determining relative signal strengths and asignal strength data repository 125 (e.g., a database, flat file orother data structure) for storing the signal strengths of the accesspoints and/or client devices on the local network 115.

In one embodiment, an access point 110 includes a roaming module 130 a,130 b (also referred to as roaming module 130) that detects clientdevice movement and triggers a determination of whether a client handoffshould occur. The determination may be performed by the calculationmodule 120. In one embedment, the access points 110 are configured touse the same SSID, security passphrase and/or security protocol (e.g.,WPA, WPA2, WEP, etc.). The access point can also include a wirelesstransceiver for transmitting and receiving wireless signals.

The following describes an example embodiment of a method fordetermining when to transfer clients from one access point to another.The values described are example values which are intended to illustratebut not to limit the disclosure.

In one embodiment, the roaming modules 130 on the access points 110communicate with each other to get the relative signal strength (RSS)between each access point. The following describes example RSS readingstaken in a three access point (AP) local network.

AP 1's Readings:

AP 1 to AP 1=100%

AP 1 to AP 2=50%

AP 1 to AP 3=0%

AP 2's Readings:

AP 2 to AP 2=100%

AP 2 to AP 3=60%

AP 2 to AP 1=50%

AP 3's Readings:

AP 3 to AP 3=100%

AP 3 to AP 2=60%

AP 3 to AP 1=0%

In one embodiment, the roaming manager 105 obtains the AP RSS readingsand attempts to build an AP relation table and identify one or moreintersect zones that denote overlap between the wireless coverage ofdifferent access points.

In an example, as illustrated in FIG. 7:

Zone 1: AP 1 intersects AP 2

Zone 2: AP 2 intersects AP 3, intersects AP 1

Zone 3: AP 3 intersects AP 2

The roaming manager 105 can use these zones to determine when to checkfor an access point handoff. For example, a client device in zone 1 isable to connect to both AP 1 and AP 2. When a client device enters thezone 1 intersect zone, the roaming manager 105 can determine whether theclient device should connect to AP 1 or AP 2 based at least partly onthe RSS of the APs.

In one embodiment, the roaming manager 105 receives a Wireless ClientTable list, which includes the RSS data from all the APs, and associatesthe clients that “intersect” multiple APs into the respective intersectzone for those APs. As clients are added to the local network 115 or aredetected to have a change in RSS greater than a particular threshold(e.g., 25%, 30%, 20%, etc.), the roaming module 130 resends data to theroaming manager 105.

In one embodiment, the roaming manager 105 calculates motion and/ortrajectory of the client. The following describes one exampleimplementation and scenario where the RSS change threshold is 25%.

-   -   1. Within a zone: A client in zone 1 is attached to AP 1 and has        a sudden 25% drop in RSS. The new RSS is now below 15%. In some        embodiments, a roaming calculation is triggered based on the RSS        drop (in this case, 25%) and/or the ending RSS (in this case,        15% being below an RSS threshold). Based on the roaming        calculation, the roaming manager 105 requests the roaming module        in AP 1 and AP 2 to perform a client transfer.    -   2. Subsequently, the client device transfers from AP 1 to AP 2.        As the client is now connected to AP 2, the roaming manager 105        can obtain the RSS strength of AP 2 at the client device as,        typically, client devices are configured to report RSS strength        to the connected access point. If the client in zone 1 gets a        significantly higher RSS than 15% (AP 1's RSS) with AP 2, then        the client is left with AP 2, as it should have a better        connection to AP 2.    -   3. If the client in zone 1 gets an even lower RSS than 15% (AP        1's RSS), then the manager 105 determines that the client is        leaving zone 1 coverage. The manager 105 can then determine,        based at least partly on the change in RSS and the AP relation        table:        -   a. If the client is moving into Zone 3 (e.g., RSS for zone 2            drops below 15% in this example implementation, indicating            the client is also moving out of zone 2)        -   b. Or else, that the client is leaving AP 1's reach and is            going away from all zones (e.g., RSS for all zones drop).

The above example illustrates how the roaming manager 105 can calculateclient movement and/or trajectory based at least partly on the RSSreported by each AP and the client device. For example, the roamingmanager 105 can determine movement from one intersection zone to anotherintersection zone. Such information can be used by the roaming manager105 to provide a more seamless experience to the user, for example, byminimizing reconnection lag when the client device transfers from one APto another. In one example, if the manager 105 determines that theclient is moving from AP 1 towards AP 2, the manager 105 can sendpre-authentication data, which can include a client identifier (e.g.,MAC address) and the client's current IP address, to AP 2 to speed upthe connection process and/or provide the same IP address to the clientas it moves across APs. In addition, in one embodiment, the manager 105can duplicate traffic going to the client device to both AP 1 and AP 2so that the client seamlessly continues receiving data as the clienttransfers across APs.

In one embodiment, the roaming manager 105, synchronizes the clientMAC/IP tables on all APs within the network. As discussed above, thiscan provide seamless transitions from one AP to another as the clientretains the same IP address. With the client information on all APswithin the network, it will be pre-authenticated to neighboring zones.

In one embodiment, when the roaming manager 105 determines the client iscloser to another AP, the manager instructs the current AP to disconnectthe client, which causes the client to automatically initiate searchingfor available APs and attempt to connect to an AP with the highest RSS.

In one embodiment, the client does not have any additional software orapplications other than conventional networking software. Asconventional networking software is generally configured to retainconnections, the client device will generally stay connected to the sameAP even when a stronger AP with a higher RSS is nearby. However, becausethe roaming manager 105 performs the determination of when to perform anAP handoff and then instructs the current AP to disconnect the client,the manager 105, in effect, forces the client to connect to the strongerAP, superseding the preference of the conventional networking softwareto remain with the same AP.

FIG. 2 illustrates a block diagram of another embodiment of the RMsystem 100. FIG. 2 illustrates that, in some embodiments, at least someof the functionality of the roaming manager 105 may be distributedacross different networking devices or other computing systems. As showin FIG. 2, instances of the roaming manager 105 a-c (or at least some ofits functionality) are found in several networking devices. For example,the signal strength data repository 125 may be stored centrally on arouter 205 while the calculation module 120 a, 120 b may be operating onone or more of the access points 110 a, 110 b. Beneficially,distributing the calculations can allow functions of the RM system 100to be performed by lower-capability processors on the networkingdevices, as each individual processor may only need to perform part ofthe calculations performed by the RM system 100 as a whole.

FIG. 3 illustrates a flow chart of an embodiment of a transfer routine300 for transferring a client device to a different access point. Insome implementations, the routine 300 is performed by embodiments of theroaming management system 100 described with reference to FIG. 1 or byone of its components, such as the roaming manager 105. For ease ofexplanation, the following describes the routine 300 as performed by theroaming manager 105. The routine is discussed in the context of anexample scenario that is intended to illustrate, but not to limit,various aspects of the RM system 100.

Beginning at block 305, the roaming manager 105 receives a roamingcalculation trigger from a first (or “source”) access point (AP 1). Forexample, AP 1 may detect that a client device connected to it now has aweak AP 1 signal and is in an intersect zone where a second AP isavailable. In some cases, the client device is configured to report theRSS of the signal it is receiving from AP 1, which allows AP 1 todetermine that the client device has a weak AP 1 signal.

At block 310, the manager 105 determines a target AP to service theclient based at least partly on roaming calculations. As discussedabove, in one embodiment, the manager 105 obtains RSS data from theaccess points. Using the RSS data, the manager 105 can estimate which APshould have the strongest signal strength at the client by comparing therelative signal strengths of the APs. In some cases, the client devicemay stay with the current AP to which it is connected (e.g., if the RSSof the current AP is above a certain threshold or is stronger than otheravailable APs), while in some cases, the client device may betransferred (e.g., another AP has a higher RSS).

At block 315, the roaming manager 105 determines whether to transfer theclient device. In one embodiment, the manager 105 transfers the deviceif it determines that the RSS for the current AP has dropped past acertain threshold (e.g., a 25% drop in signal strength). If the manager105 decides to transfer the client device, the routine proceeds to block330. If the manager 105 decides to not to transfer the client device,the routine 300 ends, thought the routine may be performed again whenanother roaming calculation is triggered (e.g., if the client devicemoves or the signal strength otherwise changes).

Optionally, at block 330, the roaming manager 105 transmits apre-authenticate command to the target AP for the client device. In oneembodiment, the command includes the client device's assigned IP addressfrom when it was connected to AP 1. Additional data may also betransmitted, such as the client device's MAC address and/or a securitytoken.

In one embodiment, the target AP is configured to provide the same IPaddress to the client device that was previously assigned to it, therebyallowing the client device to be handed off transparently toapplications operating on the client device. For example, if the user isstreaming video or browsing the Internet, packets that were addressed toits original IP address will still reach the client device because itretained its IP address even after it switches to a different AP. Thus,the user can continue using the device with minimal interruption tocurrent tasks being performed on the client device.

In one embodiment, the target AP is configured to skip normalauthentication procedures (e.g., WPA handshakes or the like) if theclient device is pre-authenticated. This can provide a more transparenthandoff process since the time taken by the client device to connect tothe target AP is minimized or at least reduced.

At block 335, the roaming manager 105 transmits a client disconnectcommand to AP 1. This causes AP 1 to disconnect the client device. Theclient device then searches for another access point to connect to.Presumably, the target AP has the highest signal strength at the clientdevice's location, thus, the client device should connect to the targetAP. As discussed above, in one embodiment, the client device includesconventional networking software that is typically configured to connectto the strongest signal for an SSID detected at the client device. Whilethe above has described block 330 as preceding block 335, in someembodiments, block 335 can be performed before or substantiallyconcurrently with block 330.

FIG. 4 illustrates a flow chart of embodiments of transfer routines 400,450 for transferring a client device from a source access point (e.g.,AP 1) to a target access point (e.g., AP 2). In some implementations,the routines 400, 450 are performed by embodiments of the roamingmanagement system 100 described with reference to FIG. 1 or by one ofits components, such as the roaming modules 130 operating on the accesspoints. For ease of explanation, the following describes the routines400, 450 as performed by roaming modules on two access points (AP 1 andAP 2). The routine is discussed in the context of an example scenariothat is intended to illustrate, but not to limit, various aspects of theRM system 100.

Beginning at block 405, AP 1 triggers a roaming calculation requestbased at least partly on the detected signal strength(s). For example,the AP 1 can determine the signal strength that it detects from theclient device and AP 1's signal strength that is reported by the clientdevice. In one embodiment, if the reported AP 1 RSS drops by a certainthreshold (e.g., 25% or more), AP 1 sends a roaming calculation requestto the roaming manager 105 (block 410). The roaming calculation requestcan be sent with various signal strength data to enable the calculationto be performed, such as the signal strengths of other APs detected byAP 1, the signal strength for AP 1 as reported by the client deviceand/or the signal strength of the client device as detected by AP 1. Insome embodiments, the signal strength data is sent separately from therequest. For example, the roaming manager 105 may be configured toperform a signal strength monitoring process by querying respective APson the LAN for the signal strength data. AP 1 may also send identifyinginformation for the client device, such as the current IP address of thedevice and its MAC address.

At block 415, assuming the roaming manager 105 decides to transfer theclient device, AP 1 receives a command from the roaming manager todisconnect the client device and disconnects the device. Subsequently,at block 417, AP 1 disconnects the client device. The routine 400 canthen end.

Meanwhile AP 2 is performing routine 450 at least partially concurrentlywith routine 400. At block 420, AP 2 receives a command from the roamingmanager to pre-authenticate the client device. AP 2 then prepares toreceive a connection request from the client device. For example, AP 2may be prepared to assign the same IP address to the client device thatit previously had when connected to AP 1. Depending on theimplementation, block 415 may be performed concurrently with block 420,block 415 may be performed first, or block 420 may be performed first.At block 425, AP 2 receives a connection request from the client deviceand connects the client to the network. Preferably, block 425 occurssoon after block 417, thereby minimizing the reconnection time for theclient device. The routine 450 can then end.

FIG. 5A illustrates an embodiment of a data structure for storing signalstrength data monitored by the RM system. In one embodiment, the system100 monitors the RSS of each reachable AP from each receiving AP. Forexample, for AP 1, the system 100 monitors the RSS of AP 2 and AP 3 asdetected from AP 1. The system 100 can also monitor the client'sreported RSS for access points reachable from its location. For example,the client may report that AP 1 has an RSS of 80 from its currentlocation, and the data is then stored by the RM system 100. The RMsystem can use the collected signal strength data in its calculations ofwhen to trigger handoffs and/or in determining where intersect zones arelocated. FIG. 5B schematically illustrates a network diagram of anexample local area network that corresponds to the signal strength dataof FIG. 5A.

FIGS. 6A-B schematically illustrates a network diagram showing an accesspoint transfer from a first access point to a second access point. InFIG. 6A, the client device is still connected to AP 1, but has movedclose enough to AP 2 that AP 2's signal strength is now greater. In FIG.6B, the RM system 100 determines that the client device should betransferred to AP 2 and forces the client to connect to AP 2. Forexample, the system 100 can cause AP 1 to disconnect the client device,which causes the client device to connect to the AP with the strongestsignal strength, AP 2.

FIG. 7 illustrates a representative network topology diagram associatedwith an AP relation table, as discussed in an example above. In FIG. 7,AP 1, AP 2 and AP 3 have overlapping wireless coverage. For example, AP1 and AP 2 overlap in Zone 1. AP 1, AP 2 and AP 3 overlap in Zone 2. AP2 and AP 3 overlap in Zone 3.

Many implementation variations are possible. For example, while theabove examples have discussed two and three AP configurations, anynumber of AP points may be used with the RM system 100. In anotherexample, the RM system may perform triangulation of the client device todetermine a physical location (e.g., when 3 or more APs are within rangeof the client device), rather than (or in addition to) relying onrelative signal strengths. In addition, while the above description hasincluded examples of operations performed with respect to “all” the APs,in some embodiments, those operations may be performed on only a subsetof the APs.

In another example variation, while the above has generally describedactions being triggered based on percentage changes in the relativesignal strengths, actions may also be triggered based on comparisons ofsignal strengths to certain thresholds. For example, if signal strengthsare measured in decibel-milliwatts (dBm), then a measurement of 30-40dBm may trigger a roaming calculation, while a measurement of 80 dBm mayindicate a good connection.

In some embodiments, the above system may be configured differently thanillustrated in the figures above. For example, various functionalitiesprovided by the illustrated modules can be combined, rearranged, added,or deleted. In some embodiments, additional or different processors ormodules may perform some or all of the functionalities described withreference to the example embodiment illustrated in the figures above.Many implementation variations are possible.

In some embodiments, the above system and its components are executed orembodied by one or more physical or virtual computing systems. Forexample, in some embodiments, a computing system, such as a server ornetworking device, that has components including a central processingunit (CPU), input/output (I/O) components, storage and memory may beused to execute some or all of the components of the above system. TheI/O components can include a display, a network connection to thenetwork, a computer-readable media drive and other I/O devices (e.g., akeyboard, a mouse, a touch screen, speakers, etc.).

Each of the processes, methods and algorithms described in the precedingsections may be embodied in, and fully or partially automated by, codemodules executed by one or more computers, computer processors, ormachines configured to execute computer instructions. The code modulesmay be stored on any type of non-transitory computer-readable storagemedium or tangible computer storage device, such as hard drives, solidstate memory, optical disc and/or the like. The processes and algorithmsmay be implemented partially or wholly in application-specificcircuitry. The results of the disclosed processes and process steps maybe stored, persistently or otherwise, in any type of non-transitorycomputer storage such as, e.g., volatile or non-volatile storage.

The various features and processes described above may be usedindependently of one another, or may be combined in various ways. Allpossible combinations and subcombinations are intended to fall withinthe scope of this disclosure. In addition, certain method, event, stateor process blocks may be omitted in some implementations. The methodsand processes described herein are also not limited to any particularsequence, and the blocks or states relating thereto can be performed inother sequences that are appropriate. For example, described tasks orevents may be performed in an order other than that specificallydisclosed, or multiple may be combined in a single block or state. Theexample tasks or events may be performed in serial, in parallel, or insome other manner. Tasks or events may be added to or removed from thedisclosed example embodiments. The example systems and componentsdescribed herein may be configured differently than described. Forexample, elements may be added to, removed from, or rearranged comparedto the disclosed example embodiments.

While certain example embodiments have been described, these embodimentshave been presented by way of example only, and are not intended tolimit the scope of the inventions disclosed herein. Thus, nothing in theforegoing description is intended to imply that any particular feature,characteristic, step, module, or block is necessary or indispensable.Indeed, the novel methods and systems described herein may be embodiedin a variety of other forms; furthermore, various omissions,substitutions and changes in the form of the methods and systemsdescribed herein may be made without departing from the spirit of theinventions disclosed herein.

What is claimed is:
 1. A wireless roaming management system, the systemcomprising: memory configured to store relative signal strength data fora plurality of access points operating on a local area network, therelative signal strength data comprising a signal strength datum fromeach one of the plurality of access points to each other one of theplurality of access points; and one or more processors configured to:receive a request for a roaming calculation for a client device from asource access point of the plurality of access points with which asignal strength from the client device has dropped below a predeterminedthreshold and triggered a roaming calculation request, wherein theclient device comprises conventional networking software and the clientdevice is distinct from the wireless roaming management system; performthe roaming calculation; determine a target access point from theplurality of access points to service the client device based at leastpartly on the stored relative signal strength data and the roamingcalculation, wherein determining the target access point comprises:determining a first intersect zone of a first set of access points inwhich the client device is located; determining a trajectory of theclient device from the first intersect zone into a second intersect zoneof a second set of access points based at least partly on changes to therelative signal strengths of the first set of access points; andselecting the target access point from the second set of access pointsbased at least partly on the trajectory; cause the client device to bedisconnected from the source access point and connected to the targetaccess point, thereby superseding preferences of the conventionalnetworking software on the client device; and transmit apre-authentication for the client device to the target access point, thepre-authentication identifying the client device to the target accesspoint and causing the target access point to skip one or moreauthentication procedures.
 2. The system of claim 1, wherein causing theclient device to be disconnected comprises transmitting a clientdisconnect command to the source access point.
 3. The system of claim 1,wherein the client device is configured to auto-initiate a connection toan access point having the strongest signal strength.
 4. The system ofclaim 1, wherein determining the target access point further comprises:comparing relative signal strengths of the second set of access points;and selecting the target access point, from the second set of accesspoints, deemed to have the strongest signal strength at the clientdevice.
 5. The system of claim 1, wherein the one or more processors arefurther configured to collect local signal strength data from each ofthe plurality of access points located on the local area network, thelocal signal strength data comprising signal strength data for one ormore other access points as observed at a respective access point.
 6. Amethod for managing wireless roaming on a local network wireless system,the method comprising: storing relative signal strength data for aplurality of access points operating on a local area network, therelative signal strength data comprising a signal strength datum fromeach one of the plurality of access points to each other one of theplurality of access points; receiving a request for a roamingcalculation for a client device from a first access point of theplurality of access points with which a signal strength from the clientdevice has dropped below a predetermined threshold and triggered aroaming calculation request, wherein the client device comprisesconventional networking software and the client device is distinct fromthe local network wireless system; performing the roaming calculation;determining a second access point from the plurality of access points toservice the client device based at least partly on the stored relativesignal strength data and the roaming calculation, wherein determiningthe second access point comprises: determining a first intersect zone ofa first set of access points in which the client device is located;determining a trajectory of the client device from the first intersectzone into a second intersect zone of a second set of access points basedat least partly on changes to the relative signal strengths of the firstset of access points; and selecting the second access point from thesecond set of access points based at least partly on the trajectory;causing the client device to be disconnected from the first access pointand connected to the second access point, thereby supersedingpreferences of the conventional networking software on the clientdevice; and transmitting a pre-authentication for the client device tothe second access point, the pre-authentication identifying the clientdevice to the second access point and causing the second access point toskip one or more authentication procedures.
 7. The method of claim 6,wherein causing the client device to be disconnected comprisestransmitting a client disconnect command to the first access point. 8.The method of claim 6, wherein the client device is configured toauto-initiate a connection to an access point having the strongestsignal strength.
 9. The method of claim 6, wherein determining thesecond access point comprises: comparing relative signal strengths ofthe second set of access points; and selecting the target access point,from the second set of access points, deemed to have the strongestsignal strength at the client device.
 10. The method of claim 6, furthercomprising collecting local signal strength data from each of theplurality of access points located on the local area network, the localsignal strength data comprising signal strength data for one or moreother access points as observed at a respective access point.